The disclosure of Japanese Patent Application No. 2000-229656 filed on Jul. 28, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to an internal combustion engine speed control system and control method for controlling an idle speed of an internal combustion engine.
2. Description of the Related Art
When an idle speed of an internal combustion engine becomes unstable, it may result in the engine stalling or induce unpleasant engine vibration. The amount of intake air during idling may also be affected by changes occurring over time in the respective parts of the internal combustion engine (for example, deposits to areas around a throttle valve change the amount of intake air and friction), causing the idle speed which has so far been stable to become unstable. To stabilize idle speed of internal combustion engines, idle speed control disclosed in, for example, Japanese Patent Application Laid-Open Publication No. HEI 6-137246, is provided.
In the idle speed control disclosed in the above publication, the engine speed goes through two different phases of control, namely, an open-loop control (with a fixed control value) provided for a period immediately after the engine has been started, which is followed by a feedback control. In such a control method, however, variations in specifications among different internal combustion engines produce differences in the idle speed during the open-loop control, making one idle speed too low and another too high. For example, if the control proceeds to the feedback phase when the idle speed is low, the idle speed is increased by a large margin, which is felt as a shock by the driver. This has been a problem to be solved.
It is therefore an object of the invention to provide an idle speed control system for an internal combustion engine that can damp shock occurring when an open-loop control is switched to a feedback control after the internal combustion engine has been started.
To achieve the foregoing object, an idle speed control system for an internal combustion engine according to a first aspect of this invention brings an engine speed, in its attempt to shift the idle speed control after the internal combustion engine has been started from an open-loop control to a feedback control, to a target speed of the feedback control in a step-by-step manner.
An idle speed control system for an internal combustion engine according to a second aspect of the invention maintains, in its attempt to shift the idle speed control after the internal combustion engine has been started from an open-loop control to a feedback control, an engine speed during the open-loop control for a predetermined period of time and, when the difference from a target speed of the feedback control becomes less than a predetermined value, shifts to the feedback control using the target speed.
An idle speed control system for an internal combustion engine according to a third aspect of the invention sets, in its attempt to shift the idle speed control after the internal combustion engine has been started from an open-loop control to a feedback control, a secondary target speed that falls within a range from an engine speed of the open-loop control to a main target speed of the feedback control and performs a control for bringing the idle speed to the secondary target speed before shifting to the feedback control using the main target speed.
An idle speed control system for an internal combustion engine according to a fourth aspect of the invention controls the idle speed according to a coolant temperature of the internal combustion engine and, when the coolant temperature becomes greater than or equal to a predetermined value, shifts the idle speed control from an open-loop control to a feedback control. If the difference between the engine speed when the coolant temperature becomes the predetermined value and a target speed of the feedback control is greater than or equal to a predetermined value, the system maintains the idle speed for a predetermined period of time before shifting to the feedback control using the target speed.